1. Field of the Invention
The present invention relates to a motor drive device and, more particularly, to a motor drive device for a brushless DC motor.
2. Description of the Prior Art
FIG. 1(a) is a circuit block diagram showing a conventional brushless motor drive device. Referring to FIG. 1(a), a motor M is a three-phase DC brushless motor having three phase coils U, V, and W. A Hall sensing circuit 11 is arranged around the motor M for detecting a position of a rotor of the motor M, thereby generating three positional detection signals HU, HV, and HW. In response to the positional detection signals HU, HV, and HW, a signal synthesizing circuit 12 generates three sinusoidal drive signals SU, SV, and SW. Subsequently, the sinusoidal drive signals SU, SV, and SW are input to a pulse width modulation (PWM) comparing circuit 13 for being individually compared with respect to a high-frequency triangular signal T generated by an oscillating circuit 14. Based on the comparison of the sinusoidal drive signals SU, SV, and SW individually with the high-frequency triangular signal T, the PWM comparing circuit 13 generates three pulse signals PU, PV, and PW to be supplied to three pre-drivers N1, N2, and N3. In response to the pulse signal PU, the pre-driver N1 generates a pair of switching signals UH and UL. In response to the pulse signal PV, the pre-driver N2 generates a pair of switching signals VH and VL. In response to the pulse signal PW, the pre-driver N3 generates a pair of switching signals WH and WL.
A three-phase switching circuit 15 has a pair of switches S1 and S2, a pair of switches S3 and S4, and a pair of switches S5 and S6, each pair being controlled by one corresponding pair of the switching signals UH and UL, VH and VL, and WH and WL. A motor drive current Im is allowed to flow from a drive voltage source Vdd to the coil U when the switch S1 becomes short-circuited and to flow from the coil U to a ground potential when the switch S2 becomes short-circuited. The motor drive current Im is allowed to flow from the drive voltage source Vdd to the coil V when the switch S3 becomes short-circuited and to flow from the coil V to the ground potential when the switch S4 becomes short-circuited. The motor drive current Im is allowed to flow from the drive voltage source Vdd to the coil W when the switch S5 becomes short-circuited and to flow from the coil W to the ground potential when the switch S6 becomes short-circuited.
For detecting the motor drive current Im, a resistor Rs is series-connected between the common connecting point of the switches S2, S4, and S6 and the ground potential. A voltage difference caused by the motor drive current Im flowing through the resistor Rs is supplied as a negative feedback to an inverting input terminal of an error amplifier EA. The error amplifier EA compares the voltage difference representative of the motor drive current Im with a current command signal Icom for generating a current error signal Ierr. Subsequently, the signal synthesizing circuit 12 adjusts the amplitudes of the sinusoidal drive signals SU, SV, and SW in accordance with the current error signal Ierr.
FIG. 1(b) is a waveform timing chart showing operations of the conventional brushless motor drive device. For the sake of simplicity, only is illustrated in FIG. 1(b) the operational waveforms associated with the coil U of the motor M since each of the phase coils U, V, and W of the motor M is operated with similar waveforms. Referring to FIG. 1(b), the pulse signal PU is generated from the comparison of the sinusoidal drive signal SU and the high-frequency triangular signal T through using the PWM comparing circuit 13. More specifically, the HIGH level of the pulse signal PU corresponds to an interval of time when the sinusoidal drive signal SU goes higher than the high-frequency triangular signal T and the LOW level of the pulse signal PU corresponds to an interval of time when the sinusoidal drive signal SU goes lower than the high-frequency triangular signal T. In response to the pulse signal PU, the pre-driver N1 generates the switching signals UH and UL for controlling the switches S1 and S2, respectively.
In order to regulate the motor drive current Im to follow the current command signal Icom, the error amplifier EA supplies the current error signal Ierr to the signal synthesizing circuit 12 for adjusting the amplitude of the sinusoidal drive signal SU. For example, when the motor drive current Im is smaller than the current command signal Icom, the current error signal Ierr controls the signal synthesizing circuit 12 to increase the amplitude of the sinusoidal drive signal SU so as to obtain a sinusoidal drive signal SU′. As clearly seen from FIG. 1(b), the sinusoidal drive signal SU′ with a larger amplitude causes the PWM comparing circuit 13 to generate a pulse signal PU′ with a larger duty ratio. In response to the pulse signal PU′ with the larger duty ratio, the three-phase switching circuit 15 causes an increase of the motor drive current Im and therefore the motor drive current Im approaches to the current command signal Icom.
However, when the difference between the motor drive current Im and the current command signal Icom becomes too large, for example, at the activation of the motor M the motor drive current Im starts from zero, the signal synthesizing circuit 12 may even generate a sinusoidal drive signal SU″ with an amplitude larger than that of the high-frequency triangular T in response to an extremely great current error signal Ierr. As a result, the PWM comparing circuit 13 generates a pulse signal PU″ with a frequency lower than that of the high-frequency triangular signal T. The low-frequency pulse signal PU″ induces a large ripple to the motor torque and deteriorates the smooth rotation of the motor M. Moreover, the low-frequency pulse signal PU″ remains at the HIGH/LOW level each cycle for a relatively long time such that the three-phase switching circuit 15 supplies the motor drive current Im in the continuous mode other than the PWM mode. The long-time continuous supply of the motor drive current Im may damage the motor M and the three-phase switching circuit 15. Also, the temperature rising caused by the large heat dissipation may trigger the thermal shutdown mechanism.